Many receivers include an in-phase/quadrature (I/Q) demodulation architecture, in which a received modulated carrier signal is simultaneously applied to an I-channel mixer and a Q-channel mixer. A local oscillator (LO) is also applied to the mixers to effect frequency conversion from a radio frequency (RF) at which the radio signal is received to an intermediate frequency (IF). In an I/Q demodulator, the LO signal that is applied to the Q-channel mixer is offset by 90° from the LO signal that is applied to the I-channel mixer.
Image rejection is one metric by which receiver system performance may be evaluated. In general, image rejection refers to the ability of the receiver to reject responses resulting from RF signals at a frequency offset from the desired RF carrier frequency by an amount equal to twice the IF of a superheterodyne receiver. For example, if the desired RF signal is at 100 megahertz (MHz), and the receiver IF is 4 MHz, then the receiver LO could be tuned to 96 MHz. However, as is well known to those skilled in the art, the receiver will also exhibit a response to undesired RF signals (i.e., image signals) at a frequency 4 MHz below the LO frequency, in this case, 92 MHz. The receiver's response to the 92 MHz signal is referred to as the image response, because the image signal resides at a frequency on the other side of the LO frequency from the desired RF carrier, and offset from the LO frequency by the magnitude of the IF. A similar image signal appears to the high side when the LO is greater than the desired RF carrier and the image frequency is larger than the LO frequency.
In a typical low-IF receiver architecture, mismatch between in-phase and quadrature phase paths causes image interference cross-talk. In many receivers, one or more strong adjacent channels may become the image interference and make reception unacceptable.